This invention relates to a novel method for etching small-ratio precisely-sized and shaped apertures into a strip of carbon steel. The etched product may be used to make shadow masks for color display cathode-ray tubes, as well as other precision-etched products.
A common type of color display cathode-ray tube comprises an evacuated glass envelope having a glass viewing window, a luminescent viewing screen supported by the inner surface of the viewing window, a formed shadow mask closely spaced from the viewing screen and an electron-gun mount assembly for generating one or more electron beams for selectively exciting the screen to luminescence. The formed shadow mask, which is a thin metal membrane having an array of precisely-sized and shaped apertures therethrough, is used as a photographic master for making the screen, and then is used, during the operation of the tube, to aid in color selection on the screen by shadowing the electron beams. For both of these functions, it is important that the apertures therein follow closely in sizes and shapes with the mask specifications.
A flat mask is ordinarily made in several steps including producing etch-resistant stencils on opposite surfaces of a strip of low-carbon steel and then etching apertures through the stencilled strip with a ferric-chloride etchant. The flat mask is then removed from the strip and formed to a desired shape. The strip is ordinarily about 0.10 to 0.20 mm (4 to 8 mils) thick, and the apertures therein may be round or slit shaped and may range in their smallest cross-sectional dimension (diameter or width) from about 0.25 mm (10 mils) to less than the thickness of the strip. In addition, the profiles of the apertures are tapered so as to reduce scattering of electrons during tube operation. Each aperture has tapered sides that terminate at its smallest periphery (diameter or width) which periphery defines the shape and size of the aperture and of the electron beamlet to pass therethrough. That smallest periphery should be precisely shaped, and the tapered surface should be as smooth as possible to aid in achieving this feature and also to reduce electron scattering.
The parameters to be controlled during the etching phase for low-carbon-steel shadow masks are well known in the art. These parameters include control of etchant temperature, Baume (specific gravity), redox potential, free-acid concentration, line speed, spray pressure and location of spray nozzles with respect to the metal strip in the etch chamber. Present factory practice is to use ferric chloride etchant with the lowest possible Baume in order to achieve the highest possible etching rate. This frequently produces rough etch resulting in high visual nonuniformity in the finished mask due to ordinary slight variations in Baume during etching. Visual nonuniformity of a shadow mask is evaluated subjectively by observing the illuminated array of apertures from the side of the mask with the larger tapers.
By rough or smooth etch, we refer to the surface roughness of the metal on the inside etched surfaces of the apertures in the shadow mask. A surface roughness equal to or less than 10 microinches (smooth etch) results in a mask with low visual nonuniformity. Increases above this value in surface roughness (rough etch) are known to contribute to a general increase in visual nonuniformity to transmitted light in the finished mask. This, in turn, degrades the ambient appearance of the phosphor screen produced with the mask, and also degrades the white uniformity of the screen in an operating picture tube.
At the present time, there are basically two types of color display cathode-ray tubes being produced. The first type is for television and general entertainment applications and is considered to have relatively low definition of the displayed video images. The second type is generally used for the display of data in the form of character, numerical and graphic information and is considered to have relatively medium or high definition. The principal factors which distinguish between these two tube types are the aperture sizes and aperture densities of the shadow masks. Generally, the second type has greater aperture density, smaller aperture sizes in the range of 0.05 to 0.15 mm (2 to 6 mils) and smaller aperture size/material thickness ratios. A practical method for distinguishing between entertainment and display type shadow masks is by the ratio of the aperture size (the smallest dimensions of the majority apertures) to the thickness of the shadow-mask membrane. In general, a mask having apertures with an aperture size/thickness ratio greater than 1.0, also referred to herein as having large-ratio apertures, describes a low-definition shadow mask used for entertainment or other low definition uses, while a mask having apertures with an aperture size/thickness ratio less than one is indicative of either a medium-or high-definition shadow mask used for data display. As the ratio of aperture size to material thickness becomes smaller, the visual nonuniformity in the shadow mask becomes greater. This is a problem for ratios in the range of about 1.0 to 2.0 and is a critical problem for etching apertures with ratios less than about 0.90, also referred to herein as small-ratio apertures.
We reported in our publication (cited below) that, when etching carbon steels with ferric chloride etchants of different specific gravities, an abrupt decrease in surface roughness occurs as the Baume is increased and/or the temperature of the etchant is decreased. Concurrent with the rapid decrease in surface roughness is the equally-sudden appearance of uniform aperture size among nearest neighbor apertures and radical improvement in the overall visual nonuniformity of the etched mask. Both of these characteristics appear to be the manifestations of a change in the reaction kinetics occurring at the surface of the low carbon steel during etching and are a function of particular Baume and temperature combinations of the etchant. We have applied our recent discovery to provide a novel method for etching small-ratio apertures in carbon steel sheet, and particularly for etching shadow masks with the majority of the apertures therein having ratios, aperture size-to-thickness, of less than about 0.90 and exhibiting minimal visual nonuniformity.